tune-*: use mcpu instead of mtune for ARM tunes

tune-*: use mcpu instead of mtune for ARM tunes

* since:
  commit cffda9a821a3b83a8529d643c567859e091c6846
  Author: Martin Jansa <Martin.Jansa@gmail.com>
  Date:   Tue Sep 11 17:05:45 2012 +0000

      arch-arm: define different ARMPKGARCH when different CCARGS are used
  we don't need to worry about e.g. cortexa7 device upgrading
  binary package from armv7a feed which would be built with
  -mcpu=cortexa15, so we can use -mcpu instead of -mtune, because
  we won't share the binary feed with MACHINEs built with different
  tune.

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>